This invention has particular but not exclusive application to glow plugs used in internal combustion engines of model airplanes used for racing.
Model airplane engines used in racing are two-cycle engines fueled with exotic fuels which burn at high temperatures. The engines conventionally turn at 20,000 rpm or higher. The engines are started by attaching a source of current to the center pin and ground of a glow plug, but immediately after they are started, the source of electrical energy is removed, and the continued running of the engine is dependent for ignition upon the retention of sufficient thermal energy from one power stroke to another by the plug. The general type of engine, but with an early type glow coil is illustrated and described in Howie, U.S. Pat. No. 2,138,301.
In conventional glow plugs now used for this purpose, a terminal pin, with an annular flange on it projecting radially from the pin intermediate the ends of the pin, is mounted in a hollow cylindrical body. The pin is insulated from the body by plastic or fiber insulators, and is held in place by a mechanical crimp or threaded sleeve. Early versions of the plug, such as are illustrated and described in Arden, U.S. Pat. No. 2,482,831 had threaded retainers, but common practice at present is to have a smooth bore and cylindrical sleeve or washer, which is held against movement out of the hollow body by a rolled or crimped edge on the body. The terminal pin stops short of the inner or nose end of the body by a substantial distance. A helically coiled resistance element extends substantially axially of the body wholly within the body except for an outer end of the element which extends radially or chordally and overlaps a radially flat inner face of the body, to which it is welded. The other end of the element is welded to the inner end of the pin.
Such plugs have a number of disadvantages. Among other things, they require the assembling of a number of small parts. They tend to reduce the compression in the cylinder, both because they do not form an entirely effective seal, and because they provide in effect a second chamber or well, in which the resistance element is located. More importantly, they expose only an end turn or two, and in any event, a small area, to the gases to be ignited, since most of the element is in the well to which the gases have no easy access. Furthermore, the geometry of the element and its connection to the body of the plug are such that the heat of combustion and pulsations of the explosions in the cylinder tend quickly to cause the element to separate at or near the exterior weld, and to displace the remaining wire of the element into the well or cavity. As a result of the latter, in practice the plug is routinely discarded after one run.
Other configurations of glow plugs have been suggested, but not for the conditions of use to which glow plugs used in model airplane racing engines are subjected. For example, U.S. Pat. to Rabezzana, Nos. 2,149,868 and Klingner, No. 2,205,145 suggest coils or loops projecting totally beyond the nose of a plug. This requires an element of a size and strength which is unacceptable in a motor of the type for which the plug of the present invention is intended. Saintsbury, U.S. Pat. No. 3,232,055 shows a circularly arranged helical element displayed at right angles to the axis of a center pin terminal, the element being supported at four quadrants, in effect making four short, relatively rigid segments.
Rademacher, U.S. Pat. No. 3,434,012 illustrates a flat spiral of ribbon resistance wire oriented at right angles to a center pin, the element being solidly supported by a fluted tubular insulator. The insulator is self-supporting and is mounted in a plug casing. The ribbon, by its nature is somewhat rigid in its "axial" direction, and without support, will tend to break up. With support, an igniter of this type has been found, in the two-cycle racing engine, to involve excessive cooling of the coil. The self-supporting core of Rademacher and those plugs which are described as prior art plugs of this nature also present certain sealing problems. Similarly, Saintsbury, U.S. Pat. No. 3,297,914 discloses a spiral heating element supported by a cross-shaped ceramic support which provides a chamber behind the heating element into which air is drawn through holes to produce a combustible mixture initially and to cool the heating element when the engine for which it is used is in operation. Such a plug has application only to jet and turbine engines in which the plug is not in a cylinder wherein a fuel mixture is compressed.
One of the objects of this invention is to provide a glow plug of the type used in small two-cycle internal combustion engines, in which more efficient ignition is provided than in glow plugs known heretofore, and which is more rugged and durable than glow plugs known heretofore. Other objects will become apparent to those skilled in the art in light of the following description and accompanying drawings.